Factors That May Influence Your Pregnancy Due Date

Were you born a big baby? Your pregnancy may last a little longer than other women. Image by Karpato Gabor.

Were you born a big baby? Your pregnancy may last a little longer than other women. Image by Karpato Gabor.

If You Were a Big Baby, You Might Deliver Later

Pounds do matter, apparently. The more you weighed as a newborn, the longer you should expect your pregnancy to last. Also, if you were overweight at the start of your pregnancy you may expect to deliver later. Researchers admitted, however, that as most of the study subjects were of normal weight, they couldn’t statistically confirm the association.

Over 35? Add A Day to Your Due Date

Were there any other predictive factors? Yes. Older mothers often had an increased length of pregnancy. Generally, if you are over the age of 35, add one day to your due date for each additional year.

Going Past 42 Weeks May Not Be a Problem

While the study has not changed the definition of preterm birth (before 37 weeks by LMP), it does indicate a possibly important shift in the cut-off standard for post-term pregnancy. Doctors now associate risks to the fetus after 42 weeks, with labor induction commonly advised before that time.

Decoded Pregnancy asked if there will likely be an increase in the use of kick counts, non-stress tests, ultrasounds, and other tools to determine if it is safe to wait.  Jukic and Wilcox replied, “Given the delicate balance between allowing a fetus all the time it needs to develop in utero, and removing a fetus from a hostile or unsupportive uterine environment, tests to determine the health of a developing fetus will always be necessary. The first step to understanding when a pregnancy is faltering is to describe healthy pregnancies as a basis of comparison. Our results provide some information about length of gestation in healthy pregnancies.

What if You Know Your Conception Date?

Fertilization generally occurs within 24 hours of ovulation. While it might be impractical for most women to pick a due date with the help of home urine tests that determine ovulation, Decoded Pregnancy asked study leaders if there is a formula that mothers who have strong intuitive feelings (or limited time with the baby’s father) can use to calculate an expected delivery day. While the researchers admitted they did not ask women when they thought they ovulated, and therefore could not attest to the accuracy of such beliefs, the average time from ovulation to delivery in the study was 268 days. Jukic and Wilson cautioned, however, that even when women know the time of ovulation, the actual length of pregnancy varies widely.

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Rates of Fetal Development Linked to Very Early Events

Among some surprises of the study were that first pregnancies did not last longer than those of experienced mothers and that having a male baby did not result in earlier delivery. Jukic and Wilson told Decoded Pregnancy, “One unexpected result was the connection between events during early pregnancy and the final length of pregnancy. For example, it takes a few days for the fertilized egg to attach to the wall of the uterus. Those that take longer to implant also took longer from implantation to birth. Embryos that are slow to get started seem also to take longer to mature. This is probably all natural variation, nothing dangerous.”

Is Absolute Due Date Determination Possible?

Calculating due dates has always been difficult. The North Carolina study reveals that the length of human pregnancy varies even more than commonly believed. By determining exactly when ovulation occurs, researchers found that a healthy pregnancy varies by 37 instead of 28 days, the range considered normal by most doctors who care for pregnant women. The timing of events in the first weeks as well as the mother’s age, pre-pregnancy and birth weight and the average length of her other pregnancies may also have predictive value when it comes to estimating when a pregnant woman will deliver.


Jukic, A, Baird, D, et al. Length of Human Pregnancy and Contributors to its Natural Variation. (2013). Human Reproduction. Accessed November 19, 2013.

Mittendorf, R, Chorzempa, L.M., et. al. Length of Pregnancy in African Americans: Validation of a New Predictive Rule. (1999). Journal of National Medical Association. Accessed November 19, 2013.

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  1. Great and interesting post. Really highlights well how there are so many factors involved in when mother’s bodies and babies will be ready for birthin’. Thanks for writing!

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